1. Field of the Invention
The present invention relates to a plant control apparatus.
2. Description of the Related Art
Existing control methods for a gasoline engine and a diesel engine set the optimum ignition timing and the optimum fuel injection timing in accordance with an engine speed, an air-fuel ratio, and the like so as to maximize the performance of the engine. It is known that the ignition timing and the injection timing are correlated with the maximum value of the cylinder pressure in each combustion cycle (hereinafter referred to as the maximum cylinder pressure). In order to prevent overloading of the engine, the maximum cylinder pressure is controlled so as not to exceed a predetermined upper limit.
FIG. 30 is a schematic graph illustrating the relationship between the ignition (injection) timing and the engine power and the relationship between the ignition (injection) timing and the maximum cylinder pressure. When the ignition (injection) timing is advanced, the power increases. However, the maximum cylinder pressure also increases. Therefore, if the ignition (injection) timing is set at the optimum timing (minimum advance for the best torque (MBT)), at which the power is at the maximum as illustrated FIG. 30, the maximum cylinder pressure may exceed the upper limit, which is determined in accordance with the strength of the engine block. In such a case, it is necessary to retard the ignition (injection) timing so that the maximum cylinder pressure does not exceed the upper limit. To prevent the maximum cylinder pressure from exceeding the upper limit with existing techniques, the ignition (injection) timing is set at a level having a predetermined margin with respect to the upper limit. This margin is determined with consideration of a production variation of the engine, a variation among cylinders, and a variation in environment. By making the margin with respect to the upper limit larger, overloading of the engine is more reliably reduced, whereas the power of the engine may decrease and the weight of the engine may increase.
Japanese Unexamined Patent Application Publication No. 2002-180879 describes a technology for controlling the maximum cylinder pressure so that the strength of the engine block can be fully exploited. This technology adjusts the fuel supply quantity, the ignition (injection) timing, and the like by monitoring the cylinder pressure of the engine using a cylinder pressure sensor. To be specific, a threshold that is lower than the upper limit is set, and if the value of the maximum cylinder pressure detected by the cylinder pressure sensor exceeds the threshold, the maximum cylinder pressure is controlled to be equal to or lower than the upper limit by reducing the fuel supply quantity.
However, when the value of the cylinder pressure is detected by the cylinder pressure sensor as described above and the maximum cylinder pressure is controlled to be equal to or lower than the upper limit by using a feedback control algorithm, a control delay inevitably occurs in the maximum cylinder pressure because the ignition (injection) timing that is calculated based on the present combustion cycle is used in the next combustion cycle. Therefore, for example, the maximum cylinder pressure may temporarily exceed the upper limit, or conversely, the power may decrease because control may be performed so as to unnecessarily reduce the maximum cylinder pressure. Such phenomena, which are against the driver's intention, often occur in a transient driving state. Accordingly, with existing technologies, the strength of the engine block is not maximally utilized, because it is necessary to control the maximum cylinder pressure to be sufficiently lower than the upper limit.
Heretofore, a problem due to a control delay in a control variable has been described by using the control of the maximum cylinder pressure of an engine as an example. However, the same problem related to a control delay may also arise when performing feedback control so as to limit a control variable of an engine to be equal to or lower than an upper limit.